Dynamics and Precursor of Delayed Fracture Studied by the Waveform Simulation of 7MHz Acoustic Emission Signals and by the Laser Excited Longitudinal Wave Velocity

摘要

With the aim of studying the precursor anddynamics of delayed fracture of high tension steel, wedeveloped two advanced elastic wave monitoring systems. Oneis a broad-band digital AE monitoring equipment consisting of amulti-resonant 7 MHz AE sensor, a 10 MHz pre-amplifier and a200 MHz A/D converter. The overall transfer function wasdetermined as a response to a pulse YAG laser break-down ofsilicon placed in slit (or crack). The break down was found tosimulate the Mode-I fracture with effective source rise time of0.035 #mu#s and successfully utilized to determine the overalltransfer function of the system. The waveform simulation of themonitored AE signals revealed the succession of fast and small(<10#mu#m) cleavage cracks with source rise time of 0.07i'5 to0.24 #mu#s in high hydrostatic stress field. In the 20 MHz laser-ultrasonic system developed, a velocity change of sphericallongitudinal (P-) wave was continuously monitored to detect theprecursor of delayed fracture. Here, the P-wave was excited bypulse YAG laser so that it propagates through the hydrostaticstress field. The velocity decreased by 100 m/s at 2.8 ks beforethe first AE signal. The velocity decrease appears to be due tohydrogen induced voids. The method has potential formonitoring the precursor of delayed fracture.